Tank Construction and Placement

The construction I used for my 500 gallon thermal storage tank is detailed below.  The tank is 88 inches wide by 40 inches deep by 36 inches high.  I choose these dimensions to fit my space, and to allow the tank bottom to use one full sheet of 4X8 plywood.   The tank would hold 550 gallons if filled to the brim.  The capacity could be increased to a little over 700 gallons by making the tank the height of a full sheet of plywood (48 inches).


I was not able to find much in the way of  how to build a plywood tank out there, so this is my cut at it.  I've described it in quite a bit of detail -- if you see places where improvements could be made, please send them in.


The structural logic for this tank design is described here.


Some comments from Tom the tank builder and others  here.


2007-2008 Winter changes to the tank here ....



I'm not a tank designer by any stretch of the imagination, so I don't guarantee that this is the best approach, or even that it will hold together.  If you have suggestions for improvements, please send them to Gary.



This is what the finished tank frame looks like without the EPDM liner, top tension tie, or insulation.


The Base

The base of the tank is built on a full sheet of 4X8 by 3/4 inch exterior plywood.  The full perimeter of the base is framed by 2X4's.  The 2X4's support the lower edges of the tank sides.



The picture shows the base plywood sheet with the 2X4's around the edge.  The 2X4's are nailed and GLUED to the base.  These 2X4's are under a lot of outward pressure from the sides and ends, so make sure they are well secured to the base plywood. 


The Sides & Ends

The sides and ends are also 3/4 inch exterior plywood.  This may be a bit thicker than needed, but the extra cost of using 3/4 instead of 1/2 is minimal.   The vertical edge of each side/end is attached to the adjacent side/end by a roughly 2X3  piece of lumber.  The sides and ends are screwed and glued to this 2X3.  The 2X3 is ripped to a nearly triangular shape so that the EPDM lining does not have to bear against a square corner. 


























End Panel:


Shows the vertical edge member being glued
and screwed to an end panel.  The edge being
clamped is the vertical edge of the end panel. 
The edge closest to the camera is the bottom edge,
which will bear against one of the 2X4's
on the tank base.


his shows the how the side and end panels are joined by the vertical 2X3.  The 2X3 is under a good deal of load from the water pressure forces on the side and end panels, so screw and glue it carefully.



An alternative to this side to end joint technique would be to let the sides run past the ends, and put and run a 2X4 down the vertical ends of the sides.  The end pieces would then be glued and screwed to the vertical 2X4.  This might well be a better way to go.



Upper Edge Frame

A frame of 2X4's surrounds the upper edge of the tank.  The 2X4's lap over each other at each corner, and the laps are glued and screwed to make strong lap joints. 



Upper Frame:

The 2X4's are glued and screwed to the sides and ends.  Where the 2X4's overlap each other at the corners, they are screwed and glued to each other -- it is important that these corner joints are strong.

Close-up of the corner joint area.


As an extra measure, a 2nd 2X4 is run across the each end and joined to the side frame 2X4's with metal splice plates.


Shows the added end frame 2X4's with metal splice plates.  This also makes for a flat surface for the cover to sit on.


Side Verticals

A vertical 2X4 is added in the center of each side to break the sides up in to two panels.


Picture showing tank in place.  The north vertical is visible to the left.  It is tied to the lower frame and upper frame with metal angle clips, and is screwed and glued to the side walls.  The picture also shows the metal tie straps on the top frame installed.


Tension Tie

The tension tie is a galvanized metal bar that ties the mid points of the two long sides together at the top frame.  It is important in that it cuts the bending load in the two top frame 2X4s by a factor of three.  Don't leave the tension tie out.


The tension tie is visible in this picture going across the top frame at the mid point of the long sides.  It is fastened to the top frame by two lag screws (1/4 inch) that are hidden by the top cap pieces in this picture. 


Installing the EPDM Liner

Carefully measure the size of the piece of liner you will need. 


Liner long dimension  = (length of side) + (tank height)*2 + (3.5)*2


Liner short dimension = (length of end) + (tank height)*2 + (3.5)*2


The 3.5 inches allows the liner to extend over the width of the top frame.


So, for this 88X40X36 inch tall tank:

Long dimension = 88 + 36*2 + 3.5*2 =  167 inches  = 14 ft

Short dimension = 40 + 36*2 + 3.5*2 = 119 inches = 10 ft


Cut out the liner to size.


Trimming the liner to size from the piece cut off of the 20 ft wide roll. 


At this point, mark each edge of the liner at its mid point with a felt tip pen -- this will allow you to line it up more easily when you put it in the tank.

Also mark the mid points of the tank upper frame 2X4's.


Lay the liner over the tank, and work it down into the tank opening.  When it hits the bottom of the tank, take your shoes off, and work from inside the tank. 

Shift the liner around until the mid points you marked on the liner line up with the mid points you marked on the top frame 2X4's.  Clamp the liner to the frame at these mid point locations (as in picture below).

Work the EPDM around until you have all the excess material in one large fold at each corner.

Make sure the EPDM has enough slack in it so that when the water fills the tank it won't be stretched.

Tack the EPDM to the top frame with a stapler (or the like) at a few places.


EPDM liner in tank, with the mid points clamped down, and the excess material worked into folds at each corner.


Install the tension tie at this point (see Tension Tie section above).  Don't leave the tension tie out.


Now install the the four boards along the edges to hold down the edges of the EPDM liner.  These edge boards hold the EPDM edges down, and provide a place for for the cover to bear against.  I used some of the left over plywood for these edge boards, but there are probably better choices that would hold up longer.




My tank sits on two thicknesses of rigid insulation board.  The top on is 2 inch thick polyisocyanurate, and the bottom one is 2 inch thick extruded polystyrene (blue) board.   I made the top layer of polyiso insulation just in case the tank temperature gets high enough to degrade polystyrene.


For the sides, I cut insulation panels from rigid foam board insulation and "glued" it to the tank with the polyurethane in a can foam material (e.g. Great Stuff).  I added one more layer of 2inch foam after the picture above was taken to bring the total R value up to about R26 -- this is probably more than is needed for an indoor tank, but mine lives outside.


Again, I used the higher temperature Polyisocyanurate insulation for the the first layer, and the extruded Polystyrene (blue) insulation for the outer two layers. 

The Polyisocyanurate insulation is sold by most lumber yards -- its the rigid foam board insulation that is usually light brown in color, and typically has aluminum foil on the faces.  It is only slightly more expensive than the more common polystyrene.



The cover is made with a 2 inch thick 4X8 sheet of extruded Polystyrene  foam board with a tempered hardboard sheet glued to the top, and EPDM glued to the bottom.  I plan to cover the whole tank with some left over fiberglass batting from a past insulation project.   This should bring the R value of the top cover to about R30.


Update: I find that it is important to have the cover well sealed to the top of the tank.   Without the tight seal, the fiberglass bating that I had over the tank got damp from the escaping vapor -- surprisingly damp in places.

I took the cover off, sealed all the joints in the edge boards that hold down the EPDM liner, and then lag screwed the insulating cover down to the perimeter frame 2X4's.  This appears to be working OK, but I think that the whole cover design could use some thought -- any ideas?


1/15/07 Update:

Most of the plumbing connections are made to the west end of the tank.  These connections are made above the EPDM lining to avoid having to penetrate the lining with pipes.  The pipes actually enter the tank through notches cut in the lid just above the EPDM liner.  Once inside the tank, the pipes are routed down to the appropriate level in the tank (high for the supply, and low for the return).  In order to be able to seal these connections up well, it is helpful to have the the west end of the tank lid (where the connections come in) sealed down permanently.   But, the rest of the tank lid still needs to be opened to provide access to the tank.   I solved this problem by cutting the notch shown in the picture below.  The notch allows the west end of the tank lid to be permanently sealed down, and the east part of the lid can still be hinged up for access using the notch as the hinge line.  The cut does not penetrate the EPDM, so no fancy sealing is needed between the fixed part of the lid and the part that can be opened.  To restore insulation, the notch cutout is just placed back in the notch.

This seems to be working well.


Notch to make the left part of the tank lid openable.  The part of the lid to the right of the notch is permanently sealed down to make it easier to do a good job of sealing the plumbing penetrations.

The return line from the collectors comes into the top of the tank just behind the saw.

The two CPVC lines going off to the house can be seen just just to the right of the tank.

The supply line to the collectors and the Taco circulation pump are on the opposite end of the tank in an insulated box.  The collector supply line enters the tank about 6 inches from the bottom, and is the only line that actually penetrates the EPDM liner.  The supply line immediately makes a 90 degree turn upward, and connects to the circulation pump.  The circulation pump is mounted such that it is below the tank waterline, and therefore retains its prime when the pump shuts down.



The supply and return line from the house entering the tank. 
These are 3/4 inch CPVC pipe.  There is a polypropylene and stainless steel strainer in the supply line (which is probably unnecessary).    I built a box around the pipes using the foam board insulation, and then filled the box up with fiberglass batting (after this picture was taken).  This arrangement has handled several -20F nights now with no problems.


The pump




The tank should be placed on a flat and level surface that will take the 4200 lb water weight.  A concrete pad, gravel pad, or well compacted soil.  I think that it is best to keep the tank on the surface, and to provide drainage to make sure that the wood stays dry.  I violated this rule with my tank, in that it is sunk in the ground about 2 ft.  In my case, the soil is dry, the tank is inside a shed that protects it from weather, and the tank bottom sits on 3 inches of open gravel plus 4 inches of insulation -- I'm still not sure this is a good idea, and I would avoid it if you can.


Suggestions for Improving the Tank Design:

While the tank design above is working out fine in use, there are always things that can be improved -- here are some suggestions from visitors, and some after thoughts from me:


From Tom, who used to be in the business of building solar heating storage tanks:

Other suggestions:


Here is one scheme to consider if you need a well insulated tank:

It takes into account the fact that if you want a well insulated tank, that the insulation quickly becomes the most expensive element of the tank.


Build the tank above ground level.  This makes it easier to incorporate cheaper fill type insulation (see below).  I actually think that the above ground is better all around.  It should be easier to build, easier to make connections, easier to inspect, easier to fix problems, and less likely to be subjected to moisture damage from the ground. 


Build the basic tank frame as shown above.


Place 1 inch of polyisocyanurate inside the plywood tank walls.  This makes a smoother surface for the EPDM to bear against, and the polyiso insulation has a high enough temperature capability to take the tank temperatures.  Putting only one inch of the insulation inside the tank keeps the reduction in tank volume to a minimum.


2/20/07 Update: unless you really need the tank volume, I would seriously consider putting 2 inches of the polyiso insulation on the inside of the plywood tank walls and floor.  This has the advantages stated above, and it provides a very good start on insulating the tank because insulation inside the tank has no thermal bridging.  When you insulate outside the tank, you have to insulate between the framing members, and these act as thermal bridges and reduce the overall R value.

Next summer, I plan to drain the tank, remove the liner, and add 2 inches of polyiso inside the tank -- I think that with this added insulation the tank temperature drop on a very cold night will get down to about 0.5F.


Place as much rigid foam board insulation under the tank base as you need to meet your heat loss requirements.  Make sure that the tank is uniformly supported over the full base area.


Build a lightweight enclosure around the sides of the tank, and fill the area between the tank frame and the enclosure with bat or loose fill insulation.  Make the space between the tank frame and the outer enclosure large enough to accommodate as much R value as you need to meet your heat loss requirements.   It will be cheaper to achieve high R values this way than stacking multiple layers of rigid foam board outside the tank.


Make the top with a sheet of EPDM glued to one or more thicknesses of rigid foam board insulation.  Again the first layer should be polyisocyanurate  insulation.   Make the cover more rigid by gluing a sheet of plywood or hardboard to the top of the top layer. 


Work out a way to seal the top down to the tank frame that prevents water vapor from getting out.  This keeps your insulation from getting wet, and eliminates a source of heat loss.


Cover the whole works with fiberglass insulation bats.


I don't think that you want to put plastic over poly film over the insulation, as this will prevent any water vapor that does get out of the tank from dissipating into the air.


Check the state of the tank structure and check that the insulation is not getting water logged from time to time.


Gary 10/25/06  Revised 10/30/06, 11/2/06